This invention is directed to polyarylates of improved color.
Polyarylates are polyesters derived from a dihydric phenol, particularly 2,2-bis(4-hydroxyphenyl)propane, also identified as bisphenol-A, and an aromatic dicarboxylic acid, particularly mixtures of terephthalic and isophthalic acids. These polyarylates are high temperature, high performance thermoplastic polymers with a good combination of thermal and mechanical properties. They also have good processability which allows them to be molded into a variety of articles.
Many processes have been described in the literature for the preparation of polyarylates. One such process is the diacetate process. In the diacetate process, a dihydric phenol is converted to its diacetate, which is then reacted with an aromatic dicarboxylic acid(s) to form the polyarylate.
However, the polyarylates produced by the diacetate process have a tendency to be colored which could make them unsuitable in some end use applications. Thus, there is a desire to improve the color of the polyarylate for such end use applications.
One known measure of polymer color which can be used as an objective standard of evaluation is the Hunter scale represented by the L, a, and b axes. L is a black-white scale representing the transmission of light through a sample, a is a red-green scale and b is a yellow-blue scale. It is preferred to obtain as close to a neutral color as possible represented by an a, b point of 0,0 on the Hunter scale. Additionally, the light transmission of the polyarylate must be considered and not adversely affected.
An attempt has been made to improve the color of melt-prepared polyarylates formed by the catalytic melt transesterification of diphenyl terephthalate and/or diphenyl isophthalate with bisphenol A. Thus, commonly assigned, copending application, U.S. Ser. No. 068,163, filed June 30, 1987, discloses including a cobalt-containing salt in the molten polymerization mixture of the above monomeric components and polymerizing in the presence of an alkali metal transesterification catalyst to yield a polyarylate which has a bluish tint which is more commercially desirable. The cobalt salts and compounds disclosed as useful in this copending application include cobalt carbonate, cobalt benzoate, cobalt phenate and cobalt acetate. The process, however, as disclosed in this prior application is not directed to forming polyarylates by the diacetate process and, thus, no predication could be made from such application as to whether the same color improvement could be achieved in polyarylates formed by the diacetate process.